Plastic container closures and, more particularly, plastic closures for carbonated beverage bottles having threaded necks are well known, having previously been disclosed, for example, in U.S. Pat. Nos. 4,310,101; 4,326,639; 4,394,918; 4,461,391; and 4,476,987. Such closures typically employ sealing discs and/or molded flanges which contact the bottle lip to reduce the loss of carbonation. Used alone, integrally molded plastic flanges have not provided the desired sealing characteristics. Although sealing discs have proved to be quite effective for reducing loss of carbonation, they are separately manufactured and inserted into a molded bottle cap, thereby increasing both the time and expense required to produce a satisfactory closure. A unitarily molded plastic bottle cap having satisfactory sealing characteristics is therefore needed.
Problems have also been encountered in manufacturing plastic closures for containers adapted to thermally insulate their contents. Such containers frequently employ plastic closures which provide sealing engagement with the lip, rim or wall of the container by means of a friction fit. This friction fit is typically achieved by means of molded threads, tapered surfaces, or separately made compressible gaskets. In order to reduce thermal conductivity through the body of such plastic closures, manufacturers have in many instances molded closures comprising an internal cavity into which a dissimilar foamed insulating material is subsequently injected. A plastic closure is therefore needed that is unitarily molded and comprises a foamed polymeric insulating core bounded by nonfoamed, outwardly-facing surface layers.
In U.S. Pat. No. 3,721,197, a one-piece plastic wad structure for use with a shotshell is formed by injecting a molten mixture of a resinous material and a foaming agent into a mold cavity, cooling the injected mixture so as to partially solidify the mixture in the cavity, and then reducing pressure in the cavity by enlarging its volume to permit a portion of the injected mixture to foam in situ.